Botunical Journal of the Linneun Societv, 73: 1-34. With 4 figures
JulyiSeptemberlOctober 1976
The taxonomy and phytogeography of brackena review
C. N . PAGE
Royal Botanic Garden, Edinburgh, Scotland
Some of the biological problems presented by bracken, Pteridium aquilinum ( L . ) Kuhn, are
posed. Its taxonomic position within the Pteridophyta and the delimitation of entities within
the genus are discussed on the basis of morphological and cytological evidence. The
geographical ranges of the various brackens world-wide are described and mapped in outline,
and emphasis placed o n reviewing t h e natural ecological role of bracken in plant communities
throughout the world. Further geographic areas where taxonomic investigation of Pteridium is
most needed are indicated, and evidence of the reproductive, dispersal, establishment,
colonizing ability and vegetative persistence of bracken is reviewed. Its palaeobiological spread,
with associated vegetational history, and the effects on this of anthropogenic influences-better
known than are comparable details for any other pteridophyte-are detailed, and the present
magnitude of the resulting bracken problem in Britain (and especially in upland Britain)
indicated.
CONTENTS
Introduction
. . . . . . . . . . . . . . . . . .
Taxonomic position in the ptcridophyta
. . . . . . . . .
Morphological evidence of phylogenetic affinities
. . . .
Cytological evidence of phylogenetic affinities
. . . . .
.
Taxonomic entitics within Pteridium and their geographical ranges
.
Geographical varieties as defined by Tryon and thcir ecology
. . . .
Subsp. uquilinum (= typicum sensu Tryon)
Subsp. caudatum
. . . . . . . . . . .
Comments o n t h e geography of Pteridium
. . . . . .
Recent taxonomic evidence from cytology and other sources
.
Natural dispersal and re-establishment of bracken
. . . . . .
Spore production, dispersal and reproduction
. . . . .
Establishment from spores
. . . . . . . . . . .
History and ecogeographical spread of bracken in Britain . . . . .
Conclusions
. . . . . . . . . . . . . . . . .
References
. . . . . . . . . . . . . . . . .
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INTRODUCTION
Bracken is the most widely distributed of the pteridophytes and, with the
possible exception of a few annual weeds, is probably the most widely
distributed of vascular plants. Bracken is present throughout the world, except
for hot and cold desert regions. I t is also one of the few pteridophytes that has
become troublesome to man. This is especially true in temperate, widely
1
2
C . N. PAGE
agriculturalized regions of the world, whereas the plant is less troublesome in
areas where the natural vegetation has been least disturbed.
To the agriculturalist, bracken presents several unique problems:
(1) Unlike most weed species, this plant has a natural world range. Bracken
was established as a member of many open forest plant communities long
before the coming of man or spread of agriculture, although the frequency of
its occurrence in many localities has increased substantially as a result of these
activities.
(2) Within this nearly world-wide range, pteridologists have long recognized
that there are distinctive differences between the brackens of many parts of the
world. Although there has been a tendency to lump all the brackens into a
single convenient species, Pteridium aquilinum (L.) Kuhn, most pteridologists
today would agree that this greatly over-simplifies the biological problem and
evidence is coming increasingly t o the fore that substantiates this view.
( 3 ) Unlike most weeds which are flowering plants, bracken is a pteridophyte, with all the peculiar advantages and disadvantages that the pteridophyte
life cycle conveys, especially concerning spore production and dissemination,
and the potential establishment of new colonies by relatively long-distance
dispersal. The fern spore is an extremely effective propagule for locating new
available habitats, and invading them, and as it takes only a single spore to
found a new colony, each bracken spore represents a large potential problem in
a very small and highly mobile form.
(4) The morphology of the pteridophyte sporophyte, vegetatively maintaining itself and perhaps spreading by means of a rhizome, endows most ferns
with an enviable longevity. In the case of bracken the rhizome is also a
subterranean and particularly hardy one and, once a clone is firmly established,
the plant has a vegetative persistence which is legendary.
To the pteridologist, few of the problems offered by bracken are unique.
Pteridium has fundamental similarities which it shares with a great many other
ferns, and it is by no means a taxonomic outlier of the group in general. The
problems and principles which apply to the taxonomy, dispersal and
distribution of Pteridium are similar in kind to, if different in degree from,
those encountered in ferns generally. The most outstanding difference is that as
a result of man’s activities, bracken has achieved a degree of reverence and
acclaim to which ferns are seldom accustomed, largely because bracken has
successfully combined a particular admixture of morphological and physiological attributes enabling it to survive in habitats which bring its presence
rather forcibly to man’s attention.
As a result of this attention, the phytogeography of Pteridiirm-its
present-day occurrence, distribution, and natural ecological role; its reproductive, dispersal, establishment and colonizing ability; its vegetative
persistence; its palaeobiological spread together with associated vegetational
history; and the effect on these aspects of anthropogenic influences-are better
known than those of any other pteridophyte. It is these phytogeographical
aspects, as well as the taxonomy of bracken, which this paper aims t o review.
The story of bracken thus provides a unique case-history of the potential
ability of a single modern fern, instructive to the agriculturalist and
p teridologist alike.
TAXONOMY A N D PHYTOCEOGRAPHY O F BRACKEN
3
TAXONOMIC POSITION I N THE PTERIDOPHYTA
The ferns are by far the largest of the four major groups of the
Pteridophyta-the ferns, the horsetails, the clubmosses and relatives, and the
psilophytes.
As a general arrangement of the overall inter-relationships and the hierarchy
of the higher groups of the pteridophyta, the excellent diagrams of PichiSermolli (1958) can be recommended for study. These show clearly the
relatively central position of the bracken family (Dennstaedtiaceae as construed
by Pichi-Sermolli) in the ferns (Filicopsida), underlining that Pteridium is by no
means a taxonomic outlier in the group.
Morphological evidence o f phylogenetic affinities
Morphological evidence of generic affinities of Pteridium suggests some
choice of detailed taxonomic treatments. Largely this is the result of
uncertainty whether various morphological traits are truly primitive or
represent evolutionary convergence in nearby groups. Holttum (1973) has
underlined the need for a new monographic treatment, based on field work, for
the genera Dennstaedtia, Microlepia and Hypolepis which may lead to new
concepts of generic boundaries in these groups, whilst their relationships to
other genera, including Puesia, Pteridium, Histiopteris and Pteris, need fresh
examination; and Mickel (1973) too has recently stressed that a monograph of
the Dennstaedtioid ferns is badly needed.
Most early authors included bracken in the genus Pteris L., on account of the
marginal position of its sori, but because of its different general habit from
other species of thus genus and its possession of a second inner (true) indusium,
it was later placed in its own genus Pteridium. Both Pteris and Pteridium were
included in the large family Polypodiaceae.
Ching (1940) was the first to break up the unwieldy family Polypodiaceae
into a number of segregate families, including the family Pteridaceae. Within
this he recognized two tribes: Pterideae (Lepidopterales) t o include Pteris and
several small genera; and Lonchitideae (Chaetopterides) to include Pteridium
along with Puesia, Lonchitis, Anisosorus and Histiopteris. He also constructed
the separate monotypic family Hypolepidaceae for the genus Hypolepis, but
noted a strong affinity between this family and the Lonchitideae of the
Pteridaceae.
Copeland (1947) accepted Ching’s family Pteridaceae but broadened it t o
contain 63 fern genera, some of which were included in separate families by
Ching, including Pteridium. He noted a probable relationship t o Puesia, which
he related t o the genera Hypolepis and Dennstaedtiu, which were also included
within this family. Holttum (1947, 1949) constructed the family Dennstaedtiaceae as a large one and included eleven diverse subfamilies; the subfamily
Pteridioideae included Pteridium, Paesia, Lonclzitis, Anisosorus and Histiopteris
along with Pteris and others. He suggested that one main line of evolution t o
Pteridium and Pteris may have been from a primitive Culcita-Dennstaedtia type
by way of Hypolepis, but he kept the latter genera in a separate subfamily
Dennstaedtioideae. Holttum (1968) noted that Pteridium agrees with Hypolepis and Dennstaedtia in its long-creeping rhizomes, the long-continued growth
4
C . N. PAGE
of the frond, grooves t o costules, costae and rachises, and in its marginal sori.
Alston (1956) however, placed Holttum’s Dennstaedtioideae with Pteridium,
in his family Dennstaedtiaceae, transferring to this family also Microlepia,
Hypolepis, Anisosorus, Lonchitis, and Histiopteris. Pichi-Sermolli ( 1959)
applied much the same treatment, including in his family Dennstaedtiaceae all
the genera of Ching’s Chaetopterides, and excluding these same genera from the
Pteridaceae. Pichi-Sermolli united his Dennstaedtiaceae along with the Dicksoniaceae and Lindsaeaceae in the order Dicksoniales, whilst grouping the
Pteridaceae with seven other families (including the Sinopteridaceae and
Adiantaceae) in a separate order Pteridales.
Mickel (1973) has defined the Dennstaedtioid ferns as genera of leptosporangiate ferns with marginal sori, two indusia, tetrahedral spores, creeping
rhizome, solenostele, rhizome indumentum of hairs, and plants which are
mainly inhabitants of wet forest. Many members of the Dennstaedtiaceae are
plants of open places in forests and forest margins and, because of their
long-creeping rhizomes and abundant branching, may form extensive colonies.
In some of the large-fronded species there is a temporary cessation of frond
development after each pair of pinnae is produced. Mickel considers Pteridium
to be a branch from a Dennstaedtia stock near t o Hypolepis and Histiopteris,
the whole group divorced from plants of pteridoid affinity.
Cytological evidence of ph y logenetic affiizities
Cytological evidence concerning fern chromosome numbers has come t o the
fore in recent years, since the work of Manton (1950), in discussion of both
species problems within fern genera (based on analysis of ploidy levels and
genome pairing in synthetic hybrids-see below) and problems of delimitation
of generic boundaries and generic affinities (chiefly resulting from chromosome
base number analysis). A substantial number of fern genera throughout the
world have now been cytologically sampled, and Walker (1973a) has pointed
out that on the basis of this evidence it is possible to make certain
generalizations in classification, ensuring that closely related genera are brought
together into one family or similar grouping, and that plants placed in
cytologically unlikely groupings are now split.
Manton (1950) was the first to report that the chromosome number ( n ) of
Pteridium was 52. On the grounds that the base chromosome number for a
genus (x) is the lowest number so far known, x = 52 was for a long time taken
as the chromosome base number for Pteridium, but we must now presume
from a count of 2n = 52 made more recently on vegetative material of
Pteridium by Love & Kjellqvist (1972) that the chromosome base number for
Pteridium is half this, i.e. x = 26, and that the polyploid series in Pteridium is
thus 2 6 , 52, 104, etc.
The known chromosome base numbers and their polyploid series reported
for Pteridium and various other fern genera with which Pteridium has been
linked on morphological evidence are presented in Table 1. I t can be seen that
the polyploid series 26, 52, 104 completely removes Pteridium from all species
of pteridaceous affinity (Pteris, Adiantum, Cheilanthes, etc.) which have
been widely shown to consist of x = 29 or 30, and to have the corresponding
polyploid series 29, 58, 116 or 30, 60, 120. Pteridizim also differs from
TAXONOMY AND PHY’TOGEOGRAPHY OF BRACKEN
5
Table 1. Known chromosome data for fern genera which have been
linked with Pteridium on morphological evidence. Information compiled from Chiarugi (1960), Fabbri (1963, 1965), Mehra & Khanna
(19591, and Walker (1958, 1962, 1966, 197313, and pers. comm.)
Chromosome base number
Genus
----_______Histiop teris
Pteridium
Paesia
-
-
26
26
52
-.
Pteris
Microlepia
30-34
(46. 47
43
Lorichitis
A nisosorus
Culcita
-
52
I-lypolepfs
Dennstaedtia
Known polyploids
(x)
49-50
58
58
60
64,66
-
86
76
100
96
104
104
104
-~
116
1 2 0 etc.
94
__
129
-.
Histiopteris, which apparently involves the series 24, 48, 96, and from
Dennstaedtia, which appears to involve at least two series of chromosome base
numbers, x = 30 to 34 and x = 46, 47 (Walker, 1966, 1973b, and pers.
comm.). The series 26, 52, 104 of Pteridium compares only, amongst all other
fern genera whose chromosomes are known, with Paesia and Hypolepis, which
are, indeed, genera which have been united with Pteridium on morphological
grounds. Counts for Paesia have all proved to be based on x = 52. Brownlie
(1954) reported Paesia scaberula (A. Rich.) Kuhn from New Zealand as n = 26,
and Walker (1966) counted n = 104 in P. viscosa St. Hil. from Jamaica. These are
presumably diploid and octoploid plants respectively of a series 26, 52, 104.
Counts for Hypolepis have similarly, in the main, proved t o be based on x =52
(or 26). Brownlie (1954, 1957, 1961) reported n =52 in Hypolepis rugulosa
(Labill.) J. Smith and n = 104 in both Hypolepis tenuifolia (Forst. f.) Bernh.
and H. purzctuta (Thunb.) Mett. from New Zealand. The number n = c. 104 has
also been reported for Hypolepis punctatu from Japan (Kurita, 1962) and for
H. repens (L.) Presl from North America (Wagner & Chen, 1964).
The weight of combined evidence from morphology and cytology thus
suggests that Pteridium can be phylogenetically closely linked only with the
genera Paesia and Hypolepis, and that the most generally useful taxonomic
treatment is probably to unite these three genera into a single fern family, for
which the name Hypolepidaceae Ching is available.
TAXONOMIC ENTITIliS WITHIN PTERIDIUM AND THEIR GEOGRAPHICAL RANGES
Pteridium has always proved a somewhat difficult genus within which to
produce a satisfactory taxonomic treatment. This difficulty stems from the
existence of two types of variation. The first is that bracken shows a number of
morphological differences in different parts of its world range, indicative of the
b
C. N. PAGE
existence of several taxonomic entities each, for the most part, having a distinct
geographical area. These morphological differences are undoubtedly coupled
also with differences in the physiology and ecology of the plants concerned,
which are of significance t o the agriculturalist. The second type of variation is
that, throughout its world range, plants of bracken show a notoriously plastic
morphology, responding readily t o differences in local environment. Some of
the more obvious of these responses are the differences in form and size of
fronds growing in sun and shade. Differences in humidity and shelter, available
moisture and no doubt in soil type probably also affect the overall appearance
of the plant. Furthermore, young plants of Pteridium are so greatly different in
appearance from the mature plants that they are seldom recognized as bracken
at all.
A further difficulty is that a genus can only be satisfactorily treated
taxonomically when all its component entities are taken into consideration
simultaneously throughout its geographical range. This ensures that similar
taxonomic criteria are uniformly applied. For a genus so geographically
widespread as Pteridium, this demands a world-wide survey. Only one such
treatment has been undertaken for Pteridium-that of Tryon (1941)-the
classic study of bracken taxonomy.
However, perhaps because of bracken’s economic significance, many other
views have been expressed in the literature both before and after Tryon’s work,
applying independent local judgements to the taxonomy of bracken in various
parts of the world. The result is that a myriad of purported minor taxonomic
categories have been described, very many of which are, in all probability,
environmentally-induced forms of no taxonomic consequence.
I t is thus difficult, short of undertaking a new monograph of the genus, t o
pick out from this literature the instances where taxonomic recognition is
clearly deserved. I have, in this paper, thus adhered t o the nomenclature and
treatment of Pteridium established by Tryon (1941). This is not to say that it is
necessarily the last word in bracken taxonomy, as several authorities have, at
times, raised the status of some or all of Tryon’s varieties to the rank of
separate species. Ching (1940), for example, considered Ptericlizim t o consist of
five or six species worldwide, and indeed some evidence is now coming to the
fore that the monotypic view of the genus Pteridium may perhaps eventually
have to be modified.
It thus seems relevant to inject a note of caution into wholesale
extrapolation of data obtained experimentally on populations of bracken from
one part of the world t o any other. *
In dealing with the varieties of bracken recognized by Tryon, I have thought
it more generally useful in this paper to concentrate on detailing geographical
and ecological aspects of the plants world-wide, whilst referring the reader t o
the original monograph (Tryon, 1941) for morphological points, as the latter
are already adequately dealt with there. Suffice t o say that the technical
characters by which the geographical varieties are recognized chiefly include
* To validate their own investigations taxonomically, experimentalists undertaking studies on bracken
are recommended to collect a pressed specimen, annotated with details of date, locality, and collector
and experimental observation, and deposit this in one of the national herbaria (in Britain: The British
Museum (Natural History), Kew or Edinburgh Royal Botanic Gardens) who can report on its considered
identification.
TAXONOMY AND PHYTOGEOGRAPHY OF BRACKEN
7
characters of the rhizome hairs, hairiness of the rachis and frond undersurface,
hair distribution, angle of the pinnules t o their mid-nerve, dissection of the
frond, shape of the ultimate segments of the frond, their division and mode of
attachment, the relative sizes of the outer and inner indusia, and the sequence
of vernation (unrolling) of the frond.
Geographical varieties as defined by Tryon and their ecology
Tryon accepted the classic view of bracken taxonomy that had been used,
amongst others, by Christensen (1906), that the genus Pteridium is monotypic,
the sole species Pteridium aquilinum occurring throughout the range of the
genus. Within this species, however, Tryon recognized a total of two subspecies
and twelve different geographic varieties.
latiusculum
aquilinurn
decompositurn
I
\
pseudocaudaturn
\
feel
wightianum
J
subsp
aquifinum
*
I
caudatum
arachnoideum
I
7
yarrabense
/
esculentum
Subsp
caudatum
A
Figure 1. Diagram t o illustrate the varieties of Pteridium aquilinum and their possible
interrelationships as indicated on morphological grounds by Tryon (1941).
Subspecies aquilinum (containing eight varieties) is chiefly north temperate
in distribution occurring throughout Central and North America, Africa and
Eurasia as far as North Queensland and the Hawaiian Islands (Fig. 2). Subspecies
caudatum (containing four varieties) is mainly Southern Hemisphere in
distribution, occurring in Central and South America, and throughout
Australasia and the South Pacific (Fig. 3).
The morphologic-taxonomic inter-relationships indicated by Tryon between
the segregate varieties are illustrated schematically in Fig. 1.
8
‘
C. N. PAGE
Subsp. ayuilinum (= typicurn sensu Tryon)
Var. aquilinum * (Fig. 2: 1 )
Var. ayuilinum ranges throughout Europe and all but the driest regions of
Africa and the adjacent is1ands.t ‘Throughout this range it is a plant of woods,
thickets and pastures, abandoned fields, open hillsides and recently burned-over
areas and is most common in dry places and on acid soils.
In Europe Valentine (1964) notes that the plant occurs almost throughout
the continent, mainly on mountains in the south, and ascending to 1800 m in
the Alps. In northern Europe Hultkn (1941) reports Pteridium extending as far
north as northern Scandinavia and Finland.
In Britain, where the ecology of Pteridium is relatively well known, it occurs
from sea level to over 600 m, and is thought to be a natural constituent of
many semi-open woodland communities such as ash, oak, oak-birch, and the
Vaccinium-rich birchwood association of the Scottish Highlands, as well as in
open habitats such as talus slopes of maritime and sub-maritime communities
(Tansley, 1953; Giminghan, 1964; McVean, 1964; Birks, 1973). It is
abundant in soils with a reaction of about pH 4.6-6.8, typically devoid of
calcium carbonate and where the exchangeable calcium content is generally low
(De Silva, 1934).
The most westerly points of the range of var. ayuilinum are in the North
Atlantic Islands. In the Azores it is reported to be common everywhere in
undisturbed soil but avoids the high cool and rainy mountains (Ward, 1970;
Wilmanns & Rasbach, 1973). In the Canary Islands it is present up t o the
uppermost limits of summer cloud belt at about 1500 m, where it forms a thick
ground layer in natural forests of Pinus canariensis prone to fire damage and is
frequent in open Erica arborea woodland at about 850 m. Here large fronds
may reach 4 m in height, and natural sporelings can be found in dry caves down
to about 2 0 0 m (Page, 1964). In Crete Brownsey & Jermy (1973) report it
present in open situations in low shrubby Maquis vegetation on schistose
siliceous soils between 300 and 1 0 0 0 m where, before the coming of man,
Quercus coccifera was probably the dominant forest species.
In Africa var. aquilinum ascends to 3000 m or more in the mountainous
regions (Tryon, 1941), where it plays a significant role in natural vegetation
communities. On Kilimanjaro (East Africa) for example, var. ayuilinum is
reported as an abundant plant with scattered fronds up to 2 m high emerging
from a vegetation of Erica arborea and Philippia excelsa with a dense ground
covering of Lycopodium clavatum in the subalpine shrub zone at about 2800
to 3100 m (Walter, 1971). In the Imatong Mountains of the Sudan it is a
ground fern of Acacia abyssinicalurginea micrantha fire-swept woodland from
1500-2500 m (Chipp, 1929) and on Mt Kenya var. ayuilinum colonizes large
tracts beyond the lower margins of the forest, and forms an abundant element
in low-rainfall Protea scrub at the upper limits of the forest around
* Referred to as subsp. and var. typicum by Tryon. The epithet aquilinum is used here and throughout
this paper to conform with the International Code of Botanical Nomenclature and modern accepted
nomenclature practice.
t The Azores, Canary Islands, Madeira, Cape Verde Islands, Madagascar, R h n i o n , Mauritius and the
Comoros. Material from Africa and the islands is often referred to var. lanuginosum Henr. (e.g.
Tardieu-Blot, 1960).
1AXONOMY AND PHYTOGEOGRAPHY OF BRACKEN
9
Figure 2. Geographical ranges of the varieties of Pteridium aquilinum suhsp. aquilinum. (For
explanation of numbers see tcxt.)
2700-3000 m (Schelpe, 1951). In the Cameroons Mountains it is similarly a
member of forest margin vegetation at 1900-2500 m (Engler, 1919).
In West Africa it occurs both near sea level and in open areas at higher
elevations. It is an abundant fern, forming thickets in open areas in ground of
mixed sand and humus on the Liberian coast, where it also secondarily
colonizes cut-over areas. It is absent from dense forest, but occurs naturally
again in upland areas above the forests (Winne, 1952; Harley, 1955).
In the southern part of the African continent var. aquilinum is relatively
common on grassy areas, on steep sunny slopes, at the fringe of ‘bush’ land,
and in open or shrub-grown gravelly ground. I t reportedly may form large
colonies and almost impenetrable masses, and is inclined to spread after fire or
in mismanaged veld (Alston, 1934; Hancock & Lucas, 1973).
n ~ ~2:2)
rn
Var. w i ~ ~ t i ~ (Fig.
Var. wightianum is an abundant plant in the Himalayan region from where it
ranges eastwards to Taiwan and southwards to Sri Lanka (Ceylon), through
Thailand and Malaysia to the Philippines, Java, Sumatra, Borneo and New
Guinea. Throughout this area it is a plant of dry hillsides, jungle clearings,
abandoned cultivation areas, volcanic craters and grassland, most usually in
sterile and often dry soil. I t occurs from 700 to 3300 m in India and up t o
2500 m in China (Tryon, 1941).
In the most north-western part of its range, Schelpe (1954) reports it as a
plant of more open scrub, in the Pine-Cedar zone of south-east Kashmir, at c.
27 50-2500 m and in forest margins and clearings of the Pine-Spruce zone at an
altitude of c. 3250 m.
In Malaya, Holttum (1961, 1968) records what is presumably var.
wightianum in Tryon’s sense (as P. aytrilinum) occurring alongside what is
presumably Tryon’s var. yarrabmsc (as P. esculentum) in Singapore, but
L
10
C. N. PAGE
further north in Malaya var. wightianum is present only in mountains whilst var.
yarrabense is usually lacking. Large plants are recorded from thickets at
1700 m on a cleared hill-top at Cameron Highlands, apparently all sterile.
Holttum adds: “I am sure that Pteridium is never present in the primitive rain
forest of the Malayan region; it occurs in clearings and in the edge of forest, but
in Malaya I have never seen it abundant except in some clearings on mountains.
I t needs a deep soil and this is not provided where ground has been cultivated
and abandoned, leaving a surface impacted by the heavy rain. Gleichenia (sensu
lato) can establish in these conditions, and forms great thickets, with a rhizome
running along or very near the ground surface; this is killed by a burn. In North
Borneo, where the Dusun people still practice shifting cultivation in hilly
country, Pteridium survives the periodic burning of secondary growth, and I
have seen it come up abundantly through the black ashes after a burn” (R. E.
Holttum, pers. comm., 1975). In the Philippines, Pteridium is reported t o be
present throughout the islands to 2000 m in open places (Copeland, 1958) but
some of this (especially at lower altitudes and on the southern islands)
probably includes material of var. yarrabense (q.v.). In Eastern Java, Schimper
(1903) notes Pteridium (presumably var. wightianum) to be one of the
dominant elements in natural open xerophilous forest of Casuarina montana
occurring between 1800 and 2800 m on exposed mountain flanks. On Mount
Wilhelm, New Guinea, Johns & Stevens (1971) report Pteridium as occurring
from 2600 to 3383 m.
Var. pubescens (Fig. 2: 3).
Var. pubescens is widely distributed throughout the American western
states, and ranges from southern Alaska southwards to California and Mexico
and east to Wyoming, Colorado and western Texas. Throughout this range it it
a common (or locally very abundant) plant in a wide variety of habitats from
moist or dry woods and clearings to open forests and open mountain slopes, it
is a common plant of pastures, thickets and woods, and is noted as regenerating
quickly after fire (Tryon, 1941; Cronquist e t al., 1972). Throughout the
western mountains it occurs amongst stands of timber such as Douglas Fir,
Aspen and Ponderosa Pine, in openings and in forested and wooded areas. It
grows in both moist and fairly dry sites and seems to prefer deep, rich, moist
soils (Dayton et al., 1937), and ranges from sea level in the Pacific Northwest
up to about 3250 m in Colorado.
The most northerly stations for var. pubescens are in the east Pacific Coastal
District of the Yukon (Hulten, 1941). In Washington State it is the most
conspicuous herbaceous species on dry sterile soils of many of the Red Fir
forests which predominate throughout the uplands of the Pacific area (Piper,
1906), and in the Washington-Oregon region it probably grows in greatest
abundance and obtains its maximum development in the Douglas Fir regions
west of the Cascade Mountains. Here Nelson (1922) notes the fronds to be very
sensitive t o cold and “a mere touch of frost” suffices to kill them. In Montana,
Standley (1920) reports it common nearly everywhere in wooded regions, in
wet thickets, or open rather dry slopes. In open places and in thin woods and
thickets the plants are small and yellowish, whilst in more shaded situations they
are large, bright green and less pubescent. It does not extend to the upper limit
TAXONOMY AND PHYTOGEOGRAPHY O F BRACKEN
11
of timber. In Nevada and Utah var. pubescens is present in open slopes and
thickets and moist woods (Tidestrom, 1925), and in California it is widely
distributed on coastal slopes and in moist places at lower elevations, and is a
common ground-cover plant in forests at higher elevations up to 3250 m.
Throughout this range it is reported from many plant communities from
coastal sage scrub and coastal woodland to subalpine forests, especially on
gentler slopes in the open pine forests, and is most abundant in the Transition
Zone, occasionally ascending into the Canadian Zone. In the Upper Sonoran
Zone it occurs in springy places and on stream banks in a rank form up to 3 m
in height whilst at higher elevations it may be as low as 6 cm (Muxleey, 1921;
Munz &Johnston, 1922; Munz & Keck, 1959; Hoover, 1966).
In Arizona Phillips (1947) has recorded it as common in the Western Yellow
Pine forests at 1500-2500 m, where it often covers the forest floor, and in Texas
too var. pubescens is restricted to higher altitudes, where it occurs in rocky
open wooded slopes and other shaded and partially shaded situations, and in
the banks and beds or alluvial soil along mountain streams at 2150-2500 m
(Palmer, 1927; Lundell, 1966; Correll & Johnston, 1970). In New Mexico,
Standley (1914) reports var. pubescens to be one of the most abundant and
widely distributed ferns, occurring in all the higher mountain ranges and
forming a conspicuous feature amongst Aspen woodland and Yellow Pine
forest. In its most southerly stations in Mexico, var. pubescens again becomes
less common, but is known from Baja California, Chihuahua, Durango and
Oaxaca. In Chihuahua, Knobloch (1942) records var. pirhescem as an
acid-loving fern on soils of pH 4.0 in open slopes under pines or along streams;
and in Oaxaca, Mickel (1965) reports it forming 3-3.5 m high fronds in
pine-oak vegetation at 1150-1500 m.
Var. feei (Fig. 2:4)
Var. feei is restricted in range to the mountains of Mexico, Guatemala and
Honduras. Little information is available on its ecology, but Tryon (1941)
reports var. feei as present in many types of open spaces up to 2800 m
throughout its range.
Var. decompositum (Fig. 2: 5)
Var. decompositum is entirely restricted in range to the Hawaiian Islands.
Throughout the Islands it is present from 300-3000 m or more, in a wide
variety of open habitats such as dry open forests, scrub and forest margins
(Hillebrand, 1888; Page, unpubl. filed notes). Skottsberg (1926) records
Pteridium on Kauai as a member of dry forest communities and of volcanic
ridges on Oahu. On Hawaii it is present both in dry regions at low altitude
(300-750 m) and at 2500 m near the summit of Mauna Loa (Fowler, 1940). On
Maui it is present on both sides of the cone and also inside the volcanic crater
of Haleakala where Robinson (1913) has reported it occurring as a small
1eath.ery form on exposed rock at 2000-3000 m, whilst exceptionally large
plants are present in woods at about 1300 m.
12
C . N . PAGE
Var. pseudocaudatum (Fig. 2:6)
Var. pseudocaudatum is a North American plant confined chiefly to the
eastern coastal plain, where it is frequent from Cape Cod to Florida and
extends far inland, though more sparsely, across the southern states to Texas.
Throughout this range it usually occurs in open woods and pastures, burnt-over
areas and abandoned fields, typically in rather dry, poor soils but occasionally
in damper and richer situations (Tryon, 1941).
In Illinois, var. pseudocaudatum is reported as a plant of oak woods
(Mohlenbrock, 1966), as an inhabitant of open forests and open meadows on
well-drained sandy soil in Louisiana (Maples & Lutes, 1966), as a plant of open
hillsides and shaded woodland in Tennessee (Shaver, 1944), dry open woods in
cherty, sandstone or granitic regions in Missouri (Palmer & Steyermark, 1932),
as a frequent plant of sandy, shaded banks and thickets in Alabama (Graves,
1920), and as a species of flat, open deciduous woodland in Georgia (Duncan,
1955). In the south-western extremity of its range in Texas, Palmer (1919)
reports var. pseudocaudatum as very common in open, sandy woods and acid
soils -werally, and Correll & Johnston (1970) record it as a plant of dry
wood,ands and thickets in the timber belt. In the extreme south of its range in
Florida, var. pseudocaudatum is recorded by Satchwell (1916) as a plant of
pine woodland.
Var. latiusculum (Fig. 2:7)
Var. htiusculum is more or less circumboreal in range, but is apparently
absent from western North America and Alaska. Its range stretches in a broad
belt across the rest of the North American continent, northern Europe (south
to Germany but not into the British Isles) and across northern Asia to Japan,
Hainan and Szechuan (Tryon, 1941). To these Chen & Huang (1974) add
Taiwan. It is absent from the ‘Real Arctic’ of both New and Old Worlds as
defined by Polunin (1951). Tryon (1941) and Boivin (1942) suggest that in
many of its more northerly stations it may have been a glacial survivor on
‘nunatak’ areas, although it seems equally possible it may have substantially
re-immigrated into these areas in post-Pleistocene times. Throughout this range
it is abundant generally in open slopes, open woodland, thickets and in damp
or more often dry, sterile and usually well-drained soil. In eastern North
America it occurs from sea level to 1500 m, and to 2700 m in the mountains of
Wyoming and Colorado respectively (Tryon, 1941).
In North America var. latiusculum ranges southward to Oklahoma and
Tennessee, with isolated outposts in other states. It is an abundant plant of
sunny, sandy slopes, low thin woods and acid soil situations generally in the
District of Columbia (Hitchcock & Standley, 1919; Maxon, 1919), is present in
Alberta and Manitoba (Boivin, 1942) and occurs abundantly in open sandy
terraces and in open birch-aspen woods west and north-west of Lake Superior
and on Manitoulin Island (Jennings, 1918; Soper, 1963). It is the only fern of
the sandy soil of pine plains where it grows especially in areas of strong light,
but is rare or absent in the dense shade of woodland of Red or White Pine in
Michigan (McFarland, 1916).
In eastern North America var. latiusculum typically inhabits open woods
TAXONOMY AND PHYTOGEOGRAPHY O F BRACKEN
13
or open slopes in moist or dry soils, where it is said t o be less aggressive than is
var. aquilinum in Europe (Webster & Steeves, 1958). Var. latiusculum is
frequent in dry, sandy or gravelly banks and borders of woods, mostly on acid
soils in the Cayuga Lake Basin area of New York State (Weigand & Eames,
1926), and grows in great abundance in the pine barrer‘s of New Jersey and
elsewhere in the coastal plain sands. Here it avoids limestone soil, but is able to
grow in only weakly acid soils such as calcareous glacial drift (Wherry, 1921). I t
forms extensive areas in dry woods and pastures, especially following fire or
disturbance, on Rhode Island (Crandall, 1965), and is common in poor acid
soils but not limestone, ascending to about 1500 m in West Virginia (Gray,
1924). I t occurs on many types of soil but especially in sandy regions, and
often forms the typical undergrowth to scrub oaks or where pine barrens are
burned in South Carolina (Bragg, 1914; Matthews, 1941), and is abundant in
thin scrub oak woods in Missouri (Standley, 1916; Palmer & Steyermark,
1932). I t is recorded from a wide range of habitats from damp, shaded ravines
to open wooded slopes and relatively dry open places in flat woods in
Tennessee (Shaver, 1944), and from dry, open woods and gravelly slopes in
Arkansas and Pennsylvania (Moore, 1940; Gruber, 1939), and is the
commonest fern in oak woods, dune meadows and open, sandy-clay soil in the
dune floras of Indiana (Peattie, 1930).
In Europe var. latiusculum is probably widely distributed in the north and
centre, but its distribution is inexactly known due to confusion with var.
aquilinum (Valentine, 1964; Jalas & Suominen, 1972). In northern Europe
Hulten (1941) reports Pteridium aquilinum (presumably containing much
material of this variety) as extending from northern Scandinavia, central
Finland and the Urals (60”N) and in Asia from Tobolsk and Tomsk provinces
from about 58”N. Throughout the U.S.S.R., Pteridium forms a broad belt and
what is presumably mostly or entirely var. luatiuscuium occurs h-om LadogaIl’men, Upper Volga and Volga-Kamon provinces, through Western and Eastern
Transcaucasia and Talysh, across the Ob region of Western Siberia and Yenisei
of Eastern Siberia to Sakhalin and Kamchatka (Komarov, 1968). To these
Hulten (1941) adds the Southern Kuriles. Throughout the U.S.S.R. bracken is
reported from coniferous and deciduous woods, coppices and slopes, often in
dry sandy ground, with preferences for calcareous soils (Komarov, 1968). The
last statement seems of particular interest for a plant which is so often calcifuge
elsewhere. In the region of the Siberian-Mongolian frontiers, Pteridium
(presumably of this variety) is reported as a rather common plant in natural
subalpine open coniferous forest and in open fir and larch woods (Printz,
1921). In the Kamchatka peninsula Pteridium of a “thick coriaceous form of
low growth” is reported t o be fairly common or constant in dry Betula ermanii
forest and has also been reported as a dwarf’ form from salt soil (presumably
alkaline) of hot springs (Hulten, 1927). Its occurrence in such situations seems
of interest when compared with the total absence of Pteridium from
comparable areas such as Iceland.
Var. latiusculum is a common fern widespread in regions of temperate
deciduous forests in Japan (Sleep, 1970). Here it is also an important member
of cool-temperate grassland associations seral towards BetulalPinus or
alternatively QuercuslFagus woodland associations, and in warm temperate
seral successions towards pine-oak and European oak associations. Such
C. N. PAGE
14
Ptcridium type grasslands are widely distributed throughout Japan and are the
result of deterioration of land by over-grazing and burning (Numata, 1974).
These Pteridium grasslands thus have a transitional character, constituting a
sera1 stage intermediate between an annual plant stage and ultimate forest.
Var. africanum (Fig. 2: 8 )
Var. africanum is an endemic taxon of south-west Africa growing in open
grassland, dry moderately light woods and in virgin forest up to 1400 m
(Tryon, 1941). Var. africanum is thus unusual amongst the geographical
segregates of Pteridium not only in being a relatively tropical plant, but also by
appearing to be more definitely a woodland plant and penetrating into virgin
forest.
Subsp. caudatum
Var. caudatum (Fig. 3:9)
Var. caudaturn is essentially a Central American-Caribbean plant in range,
present in Bermuda and southern Florida, the West Indies, throughout Central
America and into northernmost South America. Throughout this range it is a
plant of clearings, rough pastures, dry hillsides, cut-over forest land, in pine
woods, scrubland and shady rocky places, but also occasionally invading
relatively wet, marshy habitats, usually at low altitudes, but up to 2000 m in
Central America and Mexico and 3000 m in Venezuela, and from 1000 to
1300 m in the Revillagigedo Islands (Tryon, 1941).
Var. cauu‘atum is an abundant fern in dry places, ‘palm hammocks and
white sand areas’ in Florida and is the only fern of the ‘rolling pineland’ areas
of scrub or woodland with oak and other hardwoods. In dry places fronds are
under 60 cm, but may reach 2.5 m in favourable sites (Pember, 1911; Noble,
Figure 3. Geographical ranges of the varieties of Pteridium aquilinum subsp. caudatum. (For
explanation of numbers see text.)
TAXONOMY AND PHYTOGEOGRAPHY OF BRACKEN
15
1914; Long & Lakela, 1971). In its most northerly outpost in Bermuda var.
caudatum is reported as a fern of moist woods near the centre of the island and
to be abundant in marshy areas in and around sink-holes and limestone caves
(Millspaugh, 1900; Rugg, 1912), although the reported presence of Osmunda
fern in the latter localities may indicate locally more acidic conditions than
limestone would suggest. In Cuba var. caudatum is reported from sunny hill
summits around 900 m and in the Isle of Pines, 110 km south of west central
Cuba, it is the only fern of areas of light, sandy-loam soils at low altitudes,
where it forms low thickets (Britton, 1911; Jennings, 1911). Var. caudatum
forms dense masses in open sunny places on the drier leeward slopes of
Jamaica, often with other ferns characteristic of open subtropical situations
such as Histiopteris inscisa, Pityrogramma calornelanos, and various species of
Dicranopteris (Killip, 1917). Records of var. caudatum from 540 and 770 m
(T. G. Walker, pers. comm.) seem to indicate its occurrence in Jamaica at
appreciably lower altitudes and under more tropical conditions than var.
arachnoideum in the West Indies (q.v.). Elsewhere in the Caribbean var.
caudatum occurs in woodland of Caribbean Pine, either with or without fire
damage, on the islands of Abaco, Andros, Grand Bahama and New Providence,
where it may form impenetrable masses, reduced in extent or entirely
eliminated only in marshy areas or by dense broadleaf coppices (Correll, 1974).
Pteridium is noted to be amongst the plants most resistant to sulphur fumes
and amongst the first to invade quiescent volcanic areas in the Lesser Antilles
(Howard, 197 3 ) and, although apparently absent from the Yucatan peninsula,
Pteridium is a plant of dry open places in woodland at low altitude in the island
of Cozumel and in Panama (Millspaugh, 1903; Killip, 1919).
Var. arachnoideum (Fig. 3: 10)
Var. arachnoideum ranges widely from the West Indies and Cuba to
Trinidad, southern Mexico and the Galapagos and through South America.
Maxon (1924) reports it forming thickets 2.5-3.0 m high on mountain slopes
above an altitude of 1000 m in Haiti. I t has been reported from c. 1250 m in
Jamaica (T. G. Walker, pers. comm.) and hence probably generally occurs at
higher elevations than does var. caudatum in the Caribbean, and similarly is
reported as common in Central America in mountain areas (Tryon, 1941).
It is reported to occur as extensive breaks at 300-900 m in the upper parts of
the larger islands of the Galapagos (Wiggins & Porter, 1971), and is present at
400-3000 m in Peru, where it may take over entire hillsides after they are
cleared for crops, and is rampant after burning (A. F. Tryon, 1959; R. M.
Tryon, 1964).
Despite its widespread occurrence in South America, var. arachnoideum is
apparently absent from most of the Amazon Basin and from Chile and the
southern tip of the continent, and also absent from the neighbouring Falkland
Islands (Moore, 1968) and South Georgia (Greene, 1964). Its absence from the
whole of Chile has been compared with that of other pteridophytes which are
elsewhere widely distributed in South America such as Selaginella and
Anogramma, and has been attributed to a suggested inability t o cross the
barriers of the Andes and the Atacama desert (Looser, 1930, 1948). However,
the absence of bracken from the whole of the southern end of South America
16
C. N. PAGll
might equally be due solely to the absence of a member of the Pteridium
aggregate from this continent which has yet evolved the necessary temperate
physiology.
Var. esculentum (Fig. 3: 11)
Var. esculentum ranges particularly widely from Australia, New Zealand and
New Caledonia across Polynesia and Micronesia to the Society Islands where it
is a common plant of open places and scrub (Tryon, 1941).
In Australia, var. esculentum occurs in all states. I t is widely spread on
basalt, Silurian and red sand soils especially in more open forests and drier
slopes and at the edges of clearings in cool rain forests in Victoria, where fronds
may reach 3 m in height (Ewart, 1930; O’Brien, 1963; Page, unpublished field
notes). In the Sydney district and Blue Mountains, New South Wales, Pteridium
occurs in dry sclerophyll forests, damp sandy flats, sandstone gullies and at the
edge of sand dunes (Tindale, 1972) and is a common fern of dry rain-forest
margins and clearings in dry sclerophyll and a wide variety of forests up to
1300 m, especially fire-damaged Eucalyptus forests, coastal BanksiaXanthorrhoea scrub and fixed sand dunes, and is frequent at very low altitudes
and on very poor sandy soils through much of central and southern Queensland
(Page, unpubl. field notes).
In New Zealand, Pteridium is present in both the North and South Islands
from sea level to 1 2 5 0 m and is abundant except in dense forest (Dobbie,
1921). Cockayne (1921) records var. esculentum as both a coastal dune plant
on sand (such as Urtica-Muehlenbeckia scrub) and especially as a plant of
heath-a habitat which it often dominates. Such habitats have probably been
largely induced by repeated burning of forests and shrub-heath (Cockayne,
1921) and as such they are normally non-climax communities, which under
natural conditions are probably sera1 towards forest such as Kauri (B. S. Parris,
pers. comm.), although Pteridium is itself absent once the dense climax forests
become established (Dobbie, 1921). It is probably in the dying stages of the
Pteridium phase when forest is establishing that Cockayne records Pteridium to
be present in the poor light of Leptospermum low-forest where it is said to be
“virtually a scrambling liane”. In the fern-heath communities, however,
bracken occurs in nearly pure stands which are up to 1.5 m high and evergreen,
and Cockayne (1921) reports that such stands occur also on the Chatham
Islands. Levy (1923) has also noted that the invasion of grasslands by Pteridium
along with other ferns quickly establishes a secondary scrub in grassed
hill-country areas, and these are considered to mark the first stages of
succession back to forest.
Pteridium is present on Stewart Island and is reported to be on all the
outlying island groups around New Zealand from the Kermadec to the
Auckland and Campbell Islands (Atkinson, 1923), although its presence in the
Campbell Islands has been questioned by Godley (1969). I t is also present on
Lord Howe Island (Hemsley, 1896).
In New Caledonia, var. esculentum occurs generally around forest margins in
open dry woodland and other open situations in poor sandy soils (Page,
unpubl. field notes). In the New Hebrides, var. esculentum has been recorded
for Aneiteum by Kuhn (1869) and it is also present in the Solomon Islands (A.
Braithwaite, pers. comm.).
TAXONOMY AND PHYTOGEOGRAPHY O F BRACKEN
17
I t is common, especially in drier areas, on the north and north-east slopes of
the Fijian Islands, where it is often associated with Dicranopteris linearis on
poor, eroded (talasiga) soils, in areas of dry forest with an open canopy, forest
margins and open slopes up to 1200 m (Copeland, 1929; Parham, 1972; Page,
unpubl. field notes).
Pteridium is present on Tonga (Iwatsuki, 1963) but has not been reported
for the Samoas (Setchell, 1924; Christensen, 1943; Page, unpubl. field notes),
Rarotonga or Rotuma (St. John, 1954), or on Juan Fernandez or Easter Island
(Johow, 1896), and it is also absent from most of the low coral islands of
Melanesia, Polynesia and Micronesia (e.g. Copeland, 19 38; Wagner & Greuther,
1948; Skottsberg, 1953; Christensen & Skottsberg, 1953). The most easterly
occurrence of var. esculentum appears t o be on Tahiti whence the plant was
reported by Copeland (1932), but more recent collectors have not encountered
it (Page, unpubl. field notes).
Var. yarrabense (Fig. 3 : 12)
Var. yarrabense is the Old World tropical member of the caudatum group,
north from Northern Australia and Queensland t o the Philippines, Sumatra and
north-east India. Throughout its range it is normally confined to thickets,
scrubland, clearings, open slopes and at the edge of woods, from sea level up to
2500 m (Tryon, 1941). Its occurrence in Queensland, Celebes (Christ, 1898),
and the Admiralty Islands (Wagner & Greuther, 1948) suggests that it is likely
to occur also in New Guinea, although its presence in the latter area has yet to
be confirmed.
In Malaya Holttum (1968) records P. esculentum (Forster) Nakai (presumably var. yarrabense sensu Tryon) as the common bracken of the lowlands in
Malaya, which has only once been collected on the mountains (Maxwell’s Hill,
1250 m).
Comments on the geography of Pteridium
It will already be apparent that the ranges of many of the varieties, whilst
discrete over large geographical areas, overlap in several regions. I t is thus not
surprising that in many of these regions of overlap, some morphological
intergradation may occur between the overlapping taxa and, in some areas, this
intergradation may be considerable. Good examples of such behaviour are
present in North America where the ranges of several varieties touch or overlap,
and it is known that virtually all along the eastern border of the range of var.
pubescens, for example, intermediates with var. latiusculum occur, whilst var.
pseudocaudatum also intergrades to a considerable extent with var.
latiusculum. In Europe, too, from north Germany northward, it seems highly
likely thar intermediates between var. latiusculum and var. aquilinum may well
be found.
A similar situation certainly occurs in South America where specimens
intergrading between var. caudatum and var. arachnoideum are present, and
some intergradation in northern Queensland between var. esculentum and var.
yarrabense may also occur. Holttum (1968) notes that P. esculentum of Malaya
is very near P. caudatum of the American tropics; one specimen from Brazil
18
C. N. PAGE
looking quite like Malayan l? esculentum. As indicated by Tryon (1941), the
situation in China is also perhaps especially interesting, for here not only can
one find specimens of intermediate morphology between the local var.
wightianum and the circumboreal var. latiusculum, but plants intermediate
between var. wightianum and var. yarrabense undoubtedly also occur. These
latter represent intermediates not only between varieties in the sense of Tryon
but also between his two subspecies of Pteridium aquilinum.
Whether these intermediate forms represent a coming together of taxa with
areas of hybridization and possibly introgression, or whether some represent
regions of evolutionary origin of now more widespread taxa is difficult to say,
and in all probability a combination of these activities may well be present.
Recent taxonomic evidence from cytology and other sources
Cytological counts for Pteridium world-wide are few, and many of the
varieties recognized by Tryon have yet to be cytologically studied. Despite this,
cytological evidence acquired by pteridologists over the last 25 years shows
clearly that the problem of bracken taxonomy is likely to be more complex
than previously thought. Much more cytological study will, however, be
necessary to give anything like a clear picture, and Pteridium now seems ripe
for the type of hybridization and experimental cytotaxonomic investigation
which has been applied in recent years with success to genera such as
Polypodium, Dryopteris and Asplenium.
The available chromosome data for Pteridium are set out in Table 2.
Pteridium aquilinum var. aquilinum was shown to have n = 52, 2n = 104
chromosomes in British material by Manton (1950) and Wilkie (1956), whilst
similar counts for var. aquilinum have been obtained with material from West
Africa (Manton, 1959) and the Canary Islands (Page, in prep.).
Early counts for Pteridium of other varieties also showed them to have the
same chromosome number. P. aquilinum var. latiusculum from Canada and
Japan, P. aquilinum var. pseudocaudatum from North America, P. aquilinum
var. wightianum from the western Himalayas, Nepal, Sikkim, India, Sri Lanka,
and Malaysia and P. aquilinum var. esculentum from New Zealand all gave
counts of n = 52.* The number n = 52 was thus taken to represent the diploid
chromosome level in Pteridium, and counts of n = 52 with somatic counts of
2n = 104 (Wilkie, 1956; Roy, Sinha & Sakya, 1971) showed it to be sexual.
However, the occurrence of chromosome numbers as low as n = 26 in the
apparently related genus Paesiu in New Zealand (Brownlie, 1954, 1957) and
more recently the finding in material of Pteridium from south-west Spain of
n = 26 (Love & Kjellquist, 1972) suggests 26 as the diploid number for the
genus and that plants with n = 52 chromosomes are tetraploids. Furthermore,
counts on two plants of P. aquilinum var. arachnoideum originating from the
Galapagos gave differing results, one having n = 52 chromosomes and the other
2n = 208 in a root tip squash. On the basis of this evidence, both a tetraploid
and an octoploid taxon would appear to exist in var. arachnoideum. Clearly, as
most of the varieties of Bracken have been sampled, if at all, by very few
* More recent counts by T. G. Walker (pers. cornm., 1975) have since confirmed and added to these
(see Table 3).
var. esculentum
var. arachnoideum
var. arachnoideum
var. arachnoideum
presumably
var. wightianum
var. pseudocaudatum
var. wightianum
var. wightianum
*
var. aquilinum
var. aquilinum
var. aquilinum
var. aquilinum
var. la tiusculum
var. Intiusculum
var. latiusculum
Britain
Britain
West Africa
Canary Islands
Canada (southern Ontario)
Japan
Japan
Finland
U.S.A. (North Carolina)
India (western Himalayas)
Nepal (Kathmandu Valley)
Southern India
Malaya
Ceylon
Java (Mt Gedeh)
Sulawesi (=Celebes)
Spain
New Zealand
Galapagos Islands
Galapagos Islands
Trinidad
Origin of material
* On geographical grounds this could he vat. nquilinum or vat. iatiusculum.
Pteridium aquilinum
Pteridium aquilinum
Pteridium aquilinum
Pteridium aquilinum
Pteridium aquilinum
Pteridium aquilinum
Pteridium aquilinum
Pteridium aquilinum
Pteridium aquilinum
Pteridium aquilinum
Pteridium aquilinum
Pteridium aquilinum
Pteridium aquilinum
Pteridium aquilinum
Pteridium aquilinum
I'teridium aquilinum
Pteridium herediae
Pteridium aquilinum
Pteridium aquilinum
Pteridium aquilinum
Pteridium aquilinum
Species and variety
-
-
-
52
52
-
52k1
52
52
52
52
52
52
52
52
52
52
52
52
52
52
<
Chromosome number
n
2n
~
~
Tetraploid
Sexual Tetraploid
Tetraploid
Tetraploid
Tetraploid
Tetraploid
Tetraploid
Tetraploid
Tetraploid
Tetraploid
Sexual Tetraploid
Tetraploid
Tetraploid
Tetraploid
Tetraploid
Tetraploid
Diploid
Tetraploid
Octoploid
Tetraploid
Tetraploid
Ploidy
Table 2. Chromosome numbers and levels of ploidy in bracken
Manton (1950)
Wilkir (1956)
Manton (1958)
Page (in preparation)
Britton (1953)
Takahashi (1961)
Kurita (1963)
Sorsa (1961)
Wagner (1955)
Mehra & Verma (1960)
Roy, Sinha & Sakya (1971)
Abraham, Ninan & Mathew (1962)
Manton (1950)
Manton & Sledge (1954)
T. G. Walker, T11,lOI (pers. comm.)
T. G. Walker, T8.327 (pers. comm.)
Love & Kjellqvist (1972)
Browniie (1954, 1957)
Jarrett, Manton & Roy (1968)
Jarrett, Manton & Roy (1968)
T. G . Walker, T11,056 (pers. comm.)
Authority
C. N. PAGE
20
counts, very many more samples are needed from the geographic range of each
before any inference that each is of a uniform ploidy can be given. Further,
although all the tetraploid counts of n = 52 give a superficial appearance of
being the same, no irregularity of pairing has been reported, indicating that an
allopolyploid origin for each is at least a possibility. Further, there seems no
reason to suppose that every tetraploid found necessarily contains the same
genome sets; in particular, perhaps there are differences between the varieties
of subsp. aquilinum and subsp. caudatum, a question which can only be fully
resolved by an experimental hybridization study.
Of particular interest in addition to the taxa of Tryon is the Spanish diploid
and other possibly related Mediterranean taxa of Pteridium. The diploid taxon,
named Pteridium herediae (Clemente ex Colmeiro) Love & Kjellqvist comes
from the Province of Jaen, Sierra de Cazorla, where it occurs in pine forest,
dominating the forest floor in lime-rich soil (Molesworth-Allen, 1968; Love &
Kjellqvist, 1972). The occurrence of Pteridium in calcareous soils is unusual,
and contrasts sharply with the plant’s normally calcifuge habits (De Silva,
1934). P. herediae is said to differ from typical var. aquilinum, which is
normally present on siliceous soils in southern Spain, by its smaller size and
suberect tripinnate fronds. Love & Kjellqvist suggest that this plant may be
linked to forma congestum Pinto da Silva from Portugese serpentine (Pinto da
Silva, 1968) and that it seems identical on morphological grounds with P.
aquilinum var. gintlii (Rohlena) Kumm. from calcareous soils in Crna Gora and
Srbija in Yugoslavia (Rohlena, 1942), which is stated by Love & Kjellqvist also
to have 2n = 52 chromosomes. Love & Kjellqvist speculate too that P. herediae
may be widespread on calcareous soils in southern Spain and may be the same
taxon as is widespread in similar situations in the Balkan Peninsula. Brownsey
& Jermy (1973) reported finding a population of Pteridium near Potami in
Crete (1000 m) which also had pinnae held upright, the lamina densely hairy
beneath and very thick and the whole frond a greyish appearance. This
population was quite distinct in the field from the more normal stunted
bracken growing in sandy places elsewhere in Crete and agrees well with
material of plant named P. aquilinum subsp. brevipes (Tausch) Wulf noted by
Valentine (1964) as present in the Crimea and Caucasia. This apparent
extension of the range of subsp. brevipes to Crete is of interest in relation to its
possible occurrence in Turkey, and also t o the Spanish diploid.
Further investigation of Pteridium in the Mediterranean region, Africa, and
probably also in South America, China and much of south east Asia, thus seems
greatly to be desired before we can have anything like a complete pichire of the
world-wide taxonomy of Pteridium.
NATURAL
DlSPERSAL AND RE-ESTABLISHMENT O F BRACKEN
Whilst forking of the rhizome of bracken and die-away of the older parts can
be thought of as a means of local vegetative reproduction (Watt, 1940),
longer-distance dispersal into new areas is achieved in bracken, as in all ferns,
by the production and dispersal of minute airborne spores.
TAXONOMY AND PHYTOGEOGRAPHY O F BRACKEN
21
Spore product ion, dispersal and reproduction
Spore production in bracken varies enormously from locality to locality,
depending partly on habitat, and many colonies seem to be poorly if at all
fertile. Conway (1957) in a survey in the west of Scotland found the extent to
which bracken produces fertile fronds t o vary from 0-96% depending on the
station and the year.
Bracken shows a gradual decrease in fertility with increasing degrees of
shade, although vegetative growth may remain satisfactory (Dring, 1965).
Conway (1957) too has suggested that spore production is probably lower in
Pteridium in woodland areas - its original habitat. Insufficient is known,
however, about the other conditions which promote or retard fertility in
bracken growing in the open, although it seems possible, on evidence of
behaviour of other pteridophytes, that spore production may be low when the
plant is growing most freely vegetatively, and that under more severe ecological
conditions, spore production may be high. Age and maturity of the colonies
may also be a factor.
When fertile, however, spore production from bracken may be immense, and
Schwabe (1951) has noted that a single frond may well yield as much as one
gramme of spores. Conway (1957) has calculated that, with 64 spores per
sporangium, the full spore content of one frond may be up to 3 x lo8 spores.
In open habitats such production of spores may, indeed, be regularly realized.
When mature, and presumably under conditions of relatively dry weather,
the natural dehiscence mechanism ejects the spores into the air. Although it has
been estimated that this mechanism shoots the spores of ferns n o more than
1-2 cm (Ingold, 1939), once away from the frond surface natural wind currents
carry and disperse the spores. The spores of bracken are small and similar in
size to those of most other ferns (23-35 pm,McVaugh, 1935;01iver, 1967;Chen
& Huang, 1974), but for how long spores may remain airborne, and how far
they may travel, and for how long they remain viable under these conditions
are critical questions on which there exists little information, and even statistics
on the presence of airborne spores of ferns are regrettably few (Gregory, 1961).
In measurements made at Rothamsted, Hertfordshire, England, during the
summer of 1952, Gregory & Hirst (1957) reported that the mean number of
Pteridium spores taken over the whole season at 2 m above the ground was four
per cubic metre of air, and that this figure rose to a maximum of at least 36 per
cubic metre at favourabk times. No large areas of bracken were present within
several miles of the trap site but small quantities occurred within 1.6 km (but
unfortunately, it is not recorded whether these colonies were fertile).
Airborne Pteridium spores were found to be continuously present in the
atmosphere from late July until the end of September and reached peak
concentration in late August and early September. They reached their greatest
concentration in warm dry weather at Rothamsted and, similarly, Conway
(19 57) has commented upon the particularly large number of spores being
produced by bracken during the exceptionally fine summer in Britain of 1957.
Under continually dry climatic conditions, the majority of such spores
would seem likely, however, to remain largely suspended indefinitely in
turbulent air. But total airborne pollen counts are known t o be reduced
considerably by rainfall (Hamilton, 1959) and it can probably thus be inferred
22
C. N. PAGE
from this evidence that, as far as deposition of Pteridium spores in Britain is
concerned, the highest rates would probably occur during the first rainfall
following a warm dry spell, especially in late August and early September.
Establishment from spores
The general effectiveness of the fern spore as a propagule can to some extent
be judged by the speed at which appropriately adapted species of fern succeed
in colonizing naturally occurring virgin areas. The surfaces of volcanic
eruptions, being initially utterly sterile, provide a particularly vivid example of
this. On such cooled lava slopes, ferns in general have been widely recorded to
be amongst the first vascular colonizers in many parts of the world, especially
in xbtropical and tropical climates such as those of Hawaii (McCaughety,
1917), the Japanese Miyakejima and Sakurajima Islands (Yoshioka, 1974),
Rangitoto (Crookes, 1960), the West Indies (Beard, 1945), the Canary Islands
and Western Samoa (Page, unpubl. field notes) and the island volcano of
Krakatau, between Java and Sumatra. The plant succession on Krakatau has
been particularly well documented for a long period since the explosive
eruption in the island in August 1883 (e.g. Treub, 1888; Ernst, 1908; Turrill,
1935; Docters van Leeuwen, 1936) where ferns formed an especially
characteristic feature of the early stages of colonization of the cooled lava
slopes. Here, within three years of eruption, eleven species of ferns had
become firmly established, nine of which were widely spread species of the
Indo-Malayan Archipelago, and only two of which belonged to the nearby
strand-floras of the islands. Even at this early stage the number of individual
ferns was already sufficiently large to constitute a characteristic feature of the
general facies of the vegetation. A few phanerogams were first met with in
isolated pockets amongst the ferns, both on the mountain and on the beach.
These ferns included Pteridium which became established in fissures in the
cooled lava surface and, like the rest, undoubtedly immigrated into Krakatau
and established itself there from airborne spores settling upon the open habitats
provided by the bare lava slopes. Pteridium appears to have effectively
pioneered this open bare habitat, although later in the vegetative succession it
was itself largely displaced by shading and crowding from developing
angiosperm forest vegetation. I t persisted only where open areas persisted,
usually in poor dry zones or at high altitudes, and in these areas formed stands
up to 1.5 m high by 1922.
Provided other factors are not limiting, bracken spores are known to be
capable of germination under a very wide range of light intensities, varying
from poor to intense (Dubuy & Neurnbergh, 1938; Weinberg & Voeller, 1969),
and in Britain, there are numerous records of the establishment of bracken
from spores. Nearly all of these involve rapid immigration of the plants by
spores from some distance into newly exposed open virgin habitats. By
contrast, sporelings have never been found to occur in any closed vegetation
community or even within an existing bracken stand itself. Indeed Conway
(1957) reported no sign of prothalli or sporelings to occur within a bracken
colony even where the soil was “golden brown with fallen spores”. Yet pots
placed in the middle of the bracken area for some days and then brought into
the laboratory showed developing prothalli in 7-10 days. It thus seems likely
TAXONOMY A N D PHYTOGEOGRAPHY O F BRACKEN
23
that the mature bracken itself inhibits the germination of spores beneath its
own canopy, and that the primary role of the spore is thus to effect rapid
invasion of new habitats rather than to maintain the population within existing
stands..
Establishment of bracken from spores in the mortar of old brickwork has
been noted a number of times (e.g. Ridley, 1936; Thompson, 1939), but
particularly well-documented instances of invasion of bracken in England by
spores occurred in war-time bomb sites in London. Bracken was first noted to
have invaded derelict building sites when “strong clumps” had already
established in low walls as early as 1939, despite the nearest spore-producing
colony being reported to be five miles away (Lousley, 1939). Following the
bombing of London in 1940-41 air-raids, Pteridium could be found by 1943
“scattered about the city” where, by 1944, it was said to be very abundant,
especially in basements which had been exposed and not filled in. This invasion
was reported to be the result of thousands of separate colonizations by spores,
and that the damp sheltered basements provided a peculiarly favourable
habitat for establishment (Lousley, 1944).
Most of these sporelings probably occurred in mortar rubble-a habitat
which appears to be particularly favourable for establishment of bracken
sporelings everywhere. For this reason bracken sporelings can frequently be
found growing in damp sheltered situations in old mortar of walls in many
British cities as well as in country areas. These plants, with thin, delicate, soft
green hairy fronds look totally unlike mature bracken plants, and have been
described by Lousley (1936) as “an elegant fern of unusual appearance”.
In more natural habitats in Britain, sporeling bracken has been found
infrequently compared with its abundance in man-made habitats. In the wild it
has mostly been recorded from fire-damaged sites, and its appearance in similar
situations has also been noted in France and Finland (Laurent, 1914; Oinonen,
1967a). This scarcity of bracken prothalli in the wild has been partly attributed
to the activities of collembola and fungi which, it has been suggested, serve to
limit the survival of spores or development of prothalli or young sporophytes
(Conway, 1953a). Identical situations are found, however, in almost all other
pteridophytes and a more general explanation may be that the micro-habitats
required for successful establishment of fern spores in the wild are ecologically
extremely precise, and are closely governed by an intimate interplay of a
multitude of different physical conditions and biotic competition. The number
of favourable habitats is, under normal circumstances, likely to be extremely
few, and the number of spores of an appropriate species fortuitously alighting
in such situations in a viable condition and then successfully germinating,
establishing, fertilizing and producing the sporophyte, which successfully
establishes itself and ultimately matures, must be extremely few. Yet most
ferns can be induced to complete this life cycle entirely successfully once
removed from the vicissitudes of natural competition. As on lava flows, and the
mortar of recently damaged bomb sites, newly burned-over areas open up new
immediately available wild habitats in which competition pressure is initially
presumably very low indeed, and only general physical and not the underrated,
but probably far more exacting, biotic conditions are initially limiting.
Abundant evidence that burnt-over areas in Britain are especially ideal sites
for bracken colonization via spores (whether they previously carried bracken or
24
C. N. PAGE
not) has been furnished by Braid & Conway (1943), Conway (1949) and
Melville (1 965), whilst establishment of bracken sporelings in habitats recently
opened by removal of forest has been given by Benson & Blackwell (1926) and
in open landslip areas by Whyte (1930). In some of these situations,
measurements of the rate of growth of established sporelings have also been
made. All have in common that they are newly opened habitats (largely created
by man), and in colonizing such recently opened areas, Pteridium aquilinum is
an opportunistic invader closely similar in its behaviour to that of the only
other weedy pteridophyte in Britain, Equisetum arvense, the common horsetail
(Page, 1967). Conway (1949) reported sporelings of Pteridium occurring in
“quite large numbers” at Studland Heath, Dorset in September 1934 in areas of
moist heath that had been severely burnt 12 months previously. On Littleworth
Common, Melville (1965) reported finding Pteridium prothalli in mid-April in
an area which had suffered fire-damage in the previous exceptionally dry
summer of 1959. From these prothalli, sporelings up to 5 cm high had arisen
by the end of May, and after 4 months these had fronds up to 50 cm high and
approached mature fronds in shape. When the site was revisited 4%years later,
there was no apparent way of distinguishing mature sporelings from surviving
original islands of bracken except by locality, Similarly, Benson & Blackwell
(1926) made observations on the plant succession in a recently lumbered area
of Surrey. They found young plants of bracken to appear, presumably from
spore establishment, from “moist nests of Bryophyta”. The young bracken
plants advanced from these definite centres over the surrounding, usually bare,
ground, to form the locally dominant vegetation within eight years.
Measuring the rate of establishment and growth of young bracken plants in
experimental culture, it has been found that once prothalli are established,
young sporophytes can appear within as little as six weeks from sowing, and
that from this point on, the number of fronds per plant increases rapidly and
the rate of elongation of the rhizome becomes marked. Sporelings of bracken
planted out experimentally in rich soil in March had produced, by October of
the same year, up to 22 fronds and a rhizome of up to 140 cm in length. These
plants could have formed clumps 35-65 cm across by mid-summer. In
conditions of cultivation, viable spores may be produced by the second season,
and according to the authors, each Pteridium spore that germinates can thus
become a “rapidly expanding invasion” of bracken within two years (Braid &
Conway, 1943; Conway, 1949).
The available evidence thus seems to indicate consistently that bracken
establishment from spores is probably of extremely rare occurrence in truly
natural situations in Britain, but in a variety of man-induced habitats, and
especially those created by the removal of tree cover and by burning of
vegetation, bracken can colonize rapidly and extremely effectively through an
initial unseen airborne invasion, whether the area previously bore bracken or
not.
HISTORY AND ECOGEOGRAPHICAL SPREAD O F BRACKEN IN BRITAIN
The pteridophytes are paleobotanically an exceedingly old group of plants
which for at least 250 million years have very significantly contributed t o land
floras world-wide. Only with the evolution of the angiosperms have the
pteridophytes’ importance in this respect largely diminished. The lepto.
‘TAXONOMY AND PHYTOGEOGRAPHY OF BRACKEN
25
sporangiate ferns, the group to which most living species (including Pteridium)
belong, are relatively recently evolved modern ferns, which seem largely to have
appeared simultaneously with the angiosperms. The evolution of families,
genera and species has thus taken place almost exclusively amongst angiosperm-dominated vegetation, and hence most of the 12,000 or so living species
and more than 300 genera are clearly already well-attuned to the occupation of
specific ecological niches in this angiosperm-dominated world.
There are fossil records of a number of the modern fern genera in the
Tertiary period, many of which may have been widespread at the time.
Fragments of fossils attributable to Pteridium are known from Oligocene,
Miocene and Pliocene deposits in Europe where they occurred along with other
still extant European fern genera such as Dryopteris, Asplenium, Athyrium,
Woodwardia and Osmunda (Page, in prep.), and what may also be Pteridium is
known from fossil beds as far afield as Australia (Tasmania and Victoria)
(Johnston, 1888; Patton, 1928). As with most Tertiary fossil ferns, these
specimens are fragmentary, but when the relatively fragile nature of fern fronds
is considered, their susceptibility to rapid decay after the end of each season,
and the fact that Pteridium at best today grows mostly in places in which one
might imagine fossilization t o be an unlikely event, i t is perhaps not surprising
to find a scarcity of representation of it in the fossil record. However, the
occurrence of it in such widespread stations as both Britain and Australia
indicates that, even by Tertiary times, Pteridium may already have been
widespread in the world’s natural vegetation. It is thus by no means as recently
evolved a plant as its lack of very distinctive morphological forms today might
at first suggest.
In Britain, the history and spread of Pteridium from prehistoric t o present
times is known well in outline, thanks largely to palynological records of its
spores preserved in peat of different ages, and the ease by which its trilete,
tetrahedral spore can be recognized by palynologists. Macroscopic identifications also confirm its presence in Britain from various parts of the country in
the Bronze Age and Roman times (Godwin, 1956). Its spores preserved in peat
show well its early native occurrences in Britain in both interglacial and
post-glacial times as a minor element in a woodland flora, and its increase in
abundance directly due to man’s activities, especially felling and burning of the
natural forests and woodland.
Firmly establishing the presence of bracken in the interglacial period in the
British Isles is its occurrence in deposits near Gort, County Galway, Ireland.
Here Jessen, Andersen & Farrington (1959) have reported leaf fragment
material attributable to bracken from fossil leaf-beds, as well as Pteridium
spores from peat. The age of these deposits is dated as older than that of the
Riss glaciation (which began about 230,000 years ago) (Matthews, 1955).
Pteridium spores were particularly abundant at a time when pine (Pinus)
rapidly expanded, and the contemporaneous vegetation thus consisted
dominantly of pine forest with patches of smaller deciduous trees, especially
birch. The forest cover was probably not dense and many natural open areas
doubtless occurred.
Following the advance and retreat of the last of the Pleistocene glaciations,
when the climate first became warm and drier again in the Boreal period
(7600-5500 B C ) (Pennington, 1974), there was a dry land connection between
3
26
C. N. PAGE
the British Isles and continental Europe and there had also been a land
connection between Britain and Ireland. During this period, woodland
vegetation re-immigrated into the British Isles and bracken was amongst the
open woodland species that quickly returned, presumably from continental
Europe. For even in the early post-glacial period there are records of Pteridium
spores from as far north as what is now the island of Barra in the Outer
Hebrides (Blackburn, 1946) where they occurred in relatively high proportions
and probably indicate the presence of bracken in a mixed somewhat open
forest of birch (Betulu), pine, alder ( A h u s ) and hazel (Corylus). By this time
the vegetation consisted predominantly of birch woodland, especially in the
north and west of the British Isles, with pine especially in the south and east,
and elm (Ulmus) and oak (Quercus) also present in many lowland sites
(Godwin, 1940). The British vegetation can thus be visualized as one of a
mixed, probably relatively open, woodland, suitable for the natural occurrence
of bracken, and its presence in the Outer Hebrides probably indicates that
bracken already occurred widely but not obtrusively in the British vegetation
of the time.
With the coming of Mesolithic and Neolithic man, the local abundance of
Pteridium began to increase. Smith (1970) notes that spores of bracken are
widely known from Mesolithic sites in Britain and that on Dartmoor there is an
increase in their frequency towards the Boreal and Atlantic transition period,
when the climate became warmer, wetter and more oceanic over the whole of
the British Isles. Manley (1964) notes that in many parts of the British Isles
Neolithic occupation has been correlated with changes in forest history,
notably with the decline of the elm and an increase in light-loving trees, such as
ash, and weed species, Such historic heathlands as the East Anglian Brecklands
and many Irish and Scottish moorlands are believed to have originated during
Neolithic settlement, when lighter well-drained soils were apparently utilized
for shifting cultivation both in the lowlands and up to about 300 m. This
clearance, cultivation and grazing was followed by soil. erosion and podsolization and an increase in unpalatable weeds.
Further spread of bracken seems indicated following post-Neolithic disturbance of British vegetation by man (Smith, 1970; Turner, 1970), for spores
of Pteridium have been noted to increase markedly in abundance amongst a
relatively open woodland of hazel, alder, oak and elm, with some birch and
pine. At Ringinglow Bog near Sheffield, such spores were noted to increase
appreciably with a change to a wetter, more oceanic climate, together with an
associated vegetational change from a former, relatively dense, oak forest to a
lighter oak-birch woodland with high internal light (Conway, 1947).
Whilst acknowledging bracken as originally a woodland plant of moderate
shade, Tansley (1949, 1953) has pointed out that it is adaptable t o a
considerable range of light intensities. In suitable soil today bracken spreads
widely in siliceous grassland, where it attains a dominance much more
overwhelming than it does in most woodland. Pteridium is still a natural
constituent of oak and oak-birch woodland on lighter non-calcareous soils in
Britain, and the Vuccinium-rich birchwood association in Scotland (McVean,
1964; Birks, 1973).
Tansley considers that Agrostis-Festucu grassland replaces such oak-birch
woodland after man’s interference. Protected from grazing because of its
unpalatability, the bracken society which survives from the woodland
TAXONOMY AND PHYTOGEOGRAPHY O F BRACKEN
0
Figure 4 . The distribution of bracken-infested farmland in Scotland in 1957. Each spot
represents 100 acres of bracken-infested land. (Reproduced from Scottish Agricultural
Economics, 1958, by courtesy of Her Majesty’s Stationery Office, Crown copyright.
27
28
C. N. PAGE
association finds opportunity in such grassland for fresh aggressiveness a d
dominance when it is exposed to the full light of the open (Tansley, 1953).
A similar behaviour is doubtless true of bracken in many parts of the world,
once the forest cover is removed, although it seems to adapt particularly
successfully and vigorously to open situations in climates such as that of the
British Isles (especially western Scotland-Fig. 4) where there is a strong
oceanicity of climate and relatively good and frequent cloud cover. The power
of the indigenous bracken to establish and maintain itself in grassland has been
noted especially also in New Zealand where, even by 1923, out of 11 million
acres of grass-sown former forests, four million acres had already become scrub
and ‘fern’ (mostly Pteridium) (Atkinson, 1923; Levy, 1923). Levy considers
that the scrub and fern growth may mark the first stage in succession back to
forest. Similarly, Numata (1974) has commented upon the sera1 nature of
bracken towards forest in Japan.
The success of bracken between shaded and open situations is no doubt in
part due to its morphological plasticity in adapting in form to different degrees
of exposure, especially its ability to adopt a xeromorphic frond morphology in
exposed situations (Boodle, 1904; Woodhead, 1906; Druery, 19 10). Further
factors helping it are also its ability to re-establish quickly from spores on
burned sites, the ability of the underground rhizome of mature plants to avoid
damage by subsequent fires, the general unpalatability of the fronds to grazing
animals (especially sheep), dense frond-cover probably suppressive to other
small herbs, and the vegetative spread, persistence and longevity of colonies.
Watt (1940) has given an estimated age of 35 and 72 years for bracken
rhizomes at their dying end whilst Oinonen (1967b), correlating size of bracken
colonies in Finland with known rate of spread, has calculated that individual
clones have existed for over 650 years. These results are only approximations,
but give some idea of the potential success of the plant and of the time scale
involved in dealing with bracken. One of the few notable natural ecological
limitations preventing its advance is its requirement for good conditions of soil
drainage and soil aeration for active growth, for its advance is invariably
arrested by impeded drainage, although it has been reported that, even when
exposed t o marshy waterlogged conditions, its rhizomes can continue to exist
in isolated islands around embedded stones, and that such minute patches
“have great potentialities for growth” should subsequent drainage restore
suitable edaphic conditions (Poel, 1951, 1961).
CONCLUSIONS
I t is thus clear that throughout its world-wide distribution, bracken in all its
forms is a characteristic and integral part of open woodland types of
vegetation, especially where developed on relatively poor, deep, sandy soils. In
such habitats its deep-seated creeping rhizomes enable it to spread widely and
persist almost indefinitely, but in truly wild habitats it seldom becomes keenly
aggressive. Woodland or forest regeneration continues and, in natural ecological
successions to forest, bracken is characteristically little more than a pioneer
species in the vegetation, persisting only where open-canopied areas persist but
becoming shaded out and totally disappearing where ultimate forest development reaches a stage of casting sufficient shade. The removal of the forest cover
TAXONOMY AND PHYTOGEOGRAPHY O F BRACKEN
29
again creates habitats ideal for bracken invasion, whilst fires ironically not only
enable established bracken to survive and succeed in the lack of close
competition, but also create habitats ideal for bracken invasion by spores even
if the plant was absent before. I n such anthropogenic habitats, especially ones
where frequent cloud cover provides light shade as in the British Isles, bracken
then succeeds t o a degree unparalleled in natural habitats. Bracken is of limited
direct value to man or to grazing animals, but the heavy grazing pressure of
animals such as sheep both further greatly stimulates its spread and prevents
tree regeneration.
Pteridium, in all probability, first evolved in the tropics. In the palaeobotanic
past, many members of the Pteridophyta have been diverse, geographically
widespread, ecologically dominant and probably aggressive and successful at
very many times in the earth’s history and, if one takes geological time as a
whole, for a far greater span of time than that of the flowering plants. Most of
these pteridophytes probably first evolved under conditions which were
essentially tropical. In the tropics today, there exist many groups of ferns
which are morphologically diverse, many with similar and many with different
morphological, ecological, physiological and no doubt biological attributes
compared with bracken. Many of these are evolutionarily active groups, and are
able to make their presence felt very firmly against the competition and vigour
of the most aggressive of tropical vegetation-a degree of competition against
which bracken seldom finds itself in opposition. Cultivation of some of these
tropical ferns in similar competition-free environments-even under perhaps
relatively arbitrary conditions-soon confirms the view that the vigour of
Pteridium, despite its widespread temperate occurrence, when compared with
some of these tropical ferns, is not extraordinary.
The lesson to be learned from bracken is thus clear. A fern does not have to
be phenomenally vigorous in its natural habitats to succeed geographically, but
only to have a particular combination of morphological and physiological
attributes which enable it to survive well in habitats which are widely available.
Bracken has succeeded world-wide because it happened to have the right
combination of these, whilst universal, unenlightened interference by man with
natural habitats created yet more vacant situations, without regard to the
ecological implications. The problem of bracken’s spread is thus clearly one
which man has brought upon himself, and only gradual restoration of the
natural ecological balance t o the community will ensure the long-term control
of bracken.
ACKNOWLEDGEMENTS
It is a pleasure to be able to record my thanks to D. M. Henderson (Royal
Botanic Garden, Edinburgh) and Dr T. G. Walker (University of Newcastleupon-Tyne) for helpful criticism of the manuscript and assistance in many
ways, including permission to quote unpublished cytological data, and to the
numerous other colleagues who have provided valuable information and
comments on specific points.
REFERENCES
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14: 339-421.
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